How Does RF Filters Look Like?

In this article we will discuss and provide further insight about one of the very indispensable devices in all wireless systems and applications — the RF filters. We talk a lot about RF filters in literatures, about their principle, their functions and their importance. However one may find very little information concerning practical implementations of RF filter types, such as their look (implementation technologies) and why and how are they integrated into an application.

In principle, one can easily understand that a filter is a device consisting of multiple LC circuits which resonate at one particular frequency. Well it sounds pretty easy and is what we learnt in the textbook! Is this the case in reality? Here you find a picture of a teardown smartphone. Suppose a smartphone is supporting multiple frequency bands for wireless signal processing such as 3G, 4G, 5G, Wifi, Bluetooth, GPS and etc. and each of them are having different center frequencies and bandwidths.

Can you locate the ‘LC filter’ in the picture? Or could you even spot the L or C circuit?

Before we go too far in exploring current smartphone filter, lets observe the Motorola 2G phones closely. This phone is launched in the 1990s and notice. the big differences in terms of the PCB assembly, device components and packaging! Don’t forget this vintage model has less features and yet its bigger in size when compared to modern smartphone.

Back to the question, now can you identify the RF filter like the one you have seen in the textbook? If you are looking for something small then you will be surprised to know that the biggest white block on the left of the PCB is a RF filter. You will probably have many questions pop up in your mind instantly. Like why is the shape totally different from what I learnt about LC filter? Why the size is that big? Let’s unveil the mystery here.

Dielectric RF Filter vs Lumped RF Filter

This RF filter is implemented using dielectric technologies. In other words, the LC circuit is implemented using dielectric block with certain material properties and shape in order to behave as a resonant circuit and representation of LC tank circuit. Due to the application requirement of low loss and high selectivity filter performance at high frequency, it is practically impossible to realize a filter based on relatively large size lumped L or C component (the ones we see in the textbook). Take note, the higher the order of the filter, the more L and C components are needed to realize it, thus the overly large size.

A better resonator type or structure is required for miniaturization and hence in this case a dielectric resonator is chosen and implemented for the realization of RF filter. This is due to the dielectric being able to result in high Q-factor (quality) even when miniaturize. Looking at the structure and size of the filter, one can also observe that RF filter technology plays a crucial role in enabling phone miniaturization.

As the mobile phone technologies evolved from 2G to 5G, the more complicated spectrums and frequency components to be handled by a smartphone device are pushing further the limit of RF filter innovation and implementation. This explains why current implementation of RF filters are in chip packages with much smaller form factor without sacrificing technical performance as shown in the smartphone tear down pic.

Miniaturization Maxima

Furthermore, to illustrate even better how critical is it for efficient miniaturization technology, we can take a look into our 4G and upcoming 5G phones. Take a wild guess how many filters are found in them? If your guess is anywhere below 10, you are way off the mark!

A 4G phone has around 50–90 unique filters in it catering to cater for all forms of transmission, reception and communications. From 1G to 4G, from WiFi to Bluetooth, and that’s not including any streaming services or local services offered, every phone feature requires its own filter to accommodate its functions. It is estimated that a 5G phone will have at least 100+ filters for all the upcoming services it can offer.

If RF filter technology is to stick with the technology we use in the 90s, then perhaps we will have a phone the size of a book! The desperation for miniaturization of RF filters are evident. Thankfully, that’s not the case. Introducing the SAW and BAW filters.

SAW and BAW Filters

The current popular implementation technologies of RF filter for smartphone is based on acoustic wave such as SAW (surface acoustic wave) or BAW (Bulk acoustic wave) realized using piezoelectric material. Until the end of the 90s, filters used in mobile applications were based on dielectric/ceramic and SAW technologies.

SAW filters offer good miniaturization due to the acoustic wave operation but they present good performance only at low GHz range. Moreover neither ceramic nor SAW filters can be integrated on-chip because of its incompatibility with CMOS technologies which are found in more modern phone designs.

Presently, the new emergence of BAW technology has been overwhelming received due to its high performance characteristics. It is CMOS compatible and able to handle higher frequency and power handling which favors the development of 5G applications. The differences of the SAW and BAW technologies is illustrated in figure below.

Now, for anyone teaching, learning or diving into the field of RF and Microwave telecommunications, this begs a crucial question: Are the design techniques we learnt in textbooks sufficient or even closed to practical implementation? Despite the evolution of resonator type and implementation technologies? Maybe its time for the content concerning RF filters to be revised to keep up to date with the technologies moving forward.

If you find this topic is of your interest, I can discuss more about it in future articles.

Originally published at on August 8, 2021.

FILPAL designs, and builds RF and Microwave software and hardware for Cellular, Military, Academia and Test & Measurement applications.